File operations aren't Windows-friendly

It looks like `grinder_files.dart` assumes that each platform has a single path separator that's used uniformly in all paths:

https://github.com/google/grinder.dart/blob/b335e3a40a69548ac05fcfb538c8860a14d0d559/lib/grinder_files.dart#L116-L119

This is not true on Windows, where `\` and `/` may both be used as separators, including within the same path. This leads to bugs where, for example, the `copy()` function can incorrectly include the source file's directory name in the destination path.

I recommend using [the `path` package](https://pub.dev/packages/path) rather than rolling custom path manipulation.